Technical Abstract:
Saltcedar (Tamarix spp.) and giant reed (Arundo donax L.) are exotic invasive weeds along the banks of rivers and reservoirs throughout the western U.S. In the Rio Grande Basin of Texas and Mexico, these weeds compete for water resources that are critical for agricultural and urban users and that sustain native ecosystems. Because of the massive scale of weed infestations, biological control is critical for successful weed management. Biological control of saltcedar has involved successful releases of the saltcedar leaf beetle, Diorhabda spp. in Texas and many other states. Until 2007, beetles had not been released within 70 miles of the Rio Grande because of the need to evaluate their impact on exotic athel trees (Tamarix aphylla), which have been planted for shade and as a windbreak. Saltcedar beetles confined in large cages on athel or saltcedar at a field site near Kingsville, Texas killed 90-100% of the foliage on both hosts. However, in open field tests using two different beetle release strategies on mature athel, the beetles removed less than 10% of green shoot material and failed to establish populations on athel. Based in part on these results, Mexican authorities issued a statement of support for beetle releases in the “Lost River” region of the Rio Grande southeast of El Paso. The beetles were released in June 2007 and are showing signs of establishment. Giant reed is a good target for biological control because, like saltcedar, it has no close native relatives in North or South America. The biology and effectiveness of several plant-feeidng insects from Mediterranean Europe, that are known to only feed on giant reed, are being evaluated. Molecular techniques have isolated the native point of origin in southwestern Spain of most giant reed infestations in the Lower Rio Grande Basin. Ground sampling, aerial photography, and satellite-based remote sensing are being used to determine the distribution of giant reed infestations in this region.